The invention relates to velocity error compensation in a video time base corrector and particularly to a method and apparatus for separating repetitive and non-repetitive velocity errors in a video signal, and for generating separate repetitive and non-repetitive velocity error compensation signals which then are combined and used to provide optimum compensation for velocity errors experienced by the signal.
In the field of color television, and in the reproduction of color video information signals, stability requirements are one of the critical parameters which must be addressed in order to reduce time base errors, thereby preserving the necessary color quality in the reproduced color video information. One of the causes of instability is referred to as velocity error, which is produced by a variety of operating conditions some of which include geometric errors, tape tension variations and ambient temperature and humidity changes. As in the case of all time base errors affecting stability, velocity errors result from differences which occur between the effective head-to-tape speeds during the record and reproduce processes. These errors are manifested as phase shifts between color bursts from horizontal line to horizontal line, and produce a progressive phase shift of the color video signal during the interval between bursts of respective horizontal lines of the signal. This progressive phase shift is what is commonly referred to as the velocity error.
When reproducing the video signal it is necessary to compensate for these velocity errors, and this typically is achieved through the use of a time base corrector which adjusts the phase of the video information signal in accordance with the detected positional error of each horizontal synchronizing pulse and a detected phase error of each color burst. This procedure corrects the video signal at the start of each horizontal line but does not eliminate the disturbing effect caused by the erroneous progressive phase shift that occurs during the scan line and which remains uncorrected until the end of the line when the next correction is made. More particularly, in a digital time base corrector of the type wherein velocity error compensation is performed prior to the input thereto, to prevent velocity errors from occurring the digitizing sample clock must follow any variations in the off-tape video signal frequency as it is being reproduced. Any error which occurs between the clock signal and the off-tape video signal will cause the progressive phase errors of previous mention, which are commonly known in the art as hue errors and are readily visible color disturbances in the video picture. In another type of time base corrector, the velocity errors are corrected at the digital-to-analog (D/A) converter coupled downstream of the time base corrector to remove the progressive phase errors of previous mention.
Accordingly, conventional time base correctors perform velocity correction by determining the phase error which occurs at each color burst by measuring discrete samples taken at the beginning of every horizontal line during the color burst, and then applying various techniques to try to anticipate or predict the phase shift which occur between the color bursts. It is necessary to predict the velocity errors between color bursts in order to provide continuous correction across the entire line of video. However, since the only discrete point along the scan line where the error can be accurately measured is during the color burst, there is no way of accurately determining the high frequency velocity errors which occur during the scan line.
Typically, there are several techniques for trying to predict the velocity error across an entire line of video while being able to measure the actual phase error only during the color bursts. A first technique involves what is known as first order correction, wherein the phase error is measured at the beginning and end of a line during the corresponding color bursts. First order correction then assumes that the error changes linearly across the entire line of video and provides a corresponding linear velocity error correction during the line scan. Such first order correction provides relatively valid correction during the video line if the velocity error changes are not high frequency, for example, are on the order of one kilohertz. Since the sampling rate is on the order of 15,750 Hz for an NTSC color television standard, a low frequency error is sampled at a sufficiently high rate that there are minor changes in slope in the velocity error across the line. Under such conditions, first order correction works fairly efficiently.
However, there are a range of velocity errors experienced during playback of a video signal which are of high to very high frequencies. Such velocity errors may be caused by scraping and internal friction of the rotating scanner mechanism, by scraping of the tape as it is pulled past the tape guides, and by impact errors caused when the various erase, record and reproduce heads strike the tape during the reproduce process. Such phenomena cause vibration in the tape which actually moves the tape back and forth across the reproduce head causing high frequency timing errors between the head and tape. The impact errors caused by a head striking the tape occur at some of the highest frequencies and exemplify high velocity errors which cannot be corrected by first order correction techniques.
Accordingly, a more sophisticated technique for compensating velocity errors involves second order correction, wherein the curvature of the phase error which occurs between bursts also is predicted. In such second order correction techniques, instead of looking at only the two bursts at the beginning and end of the line being corrected, three or more bursts encompassing the line being corrected are sampled to provide additional information which then is used to predict the curvature of the error with more accuracy. Sampling the velocity error at more than two bursts and performing arithmetic which predicts the direction and the extent of the curvature, provides an error correction signal which more efficiently reduces the high frequency velocity error along the middle of the line being corrected.
However, even with second order correction, very high frequency velocity errors such as those generated by the sudden impact of a head striking the tape, are not precisely corrected since there still is insufficient information available from even three or more bursts when the errors change rapidly during the interval of a scan line being corrected. That is, it has been found that more information which occurs in close spatial distance from the area being corrected is required in order to correct the higher frequency velocity errors. None of the first or second order techniques of previous mention, or any other higher order technique that simply uses more and more burst information, are able to provide the specific information required to enable precise correction of high frequency velocity errors occurring in the middle of a scan line. It follows that it would be highly desirable to provide some technique for generating information which more accurately defines the high frequency velocity errors which occur along the middle of a scan line, and for supplying the information in a manner to enable such velocity error correction. More particularly, it is highly desirable to provide a technique for enabling the precise compensation of very high frequency repetitive velocity errors known as impact errors caused when a rotating head strikes the tape during the reproduction process.
In addition, time base correctors presently in use provide velocity error correction for the full range of low to very high frequency errors, utilizing the common first and second order correction techniques of previous mention. However as discussed above, very high frequency impact velocity errors, for example, cannot be precisely corrected by techniques which are adequate for correcting low and high frequency random velocity errors. Typically, present velocity compensating techniques attempt to correct both repetitive and random velocity errors with a single configuration of the second order correction technique. However, since repetitive and random velocity errors have different characteristics, the common correction techniques provide at best only a compromised correction of each error. Accordingly, it also would be highly desirable to separate repetitive velocity errors from random velocity errors, whereby more accurate correction techniques tailored to each type of velocity error may be applied specifically to the respective velocity errors. More particularly, since the very high frequency impact velocity errors are particularly visible, it would be highly desirable to separate impact velocity errors from random velocity errors whereby the former may be corrected by a velocity compensation circuit optimized for impact error characteristics.
To illustrate the detrimental effects of the very high frequency impact errors, when re-recording multiple generations of a recording, random velocity errors build up gradually due to their random characteristic; that is, random errors increase approximately 1.4 times for each recording generation. However, repetitive velocity errors such as impact errors have the same time base error, that is, are coherent with vertical sync, and thus double in amplitude with each generation. It may be seen that the repetitive characteristic of impact velocity errors therefore can lead to undesirably large velocity errors which, in turn, cause very visible color hue disturbances in the video picture if not properly compensated.
Accordingly, the present invention overcomes the disadvantages of present compromise velocity error compensating techniques, by providing a method and apparatus for separating high frequency repetitive velocity errors, such as those caused by head impact, from random velocity errors, wherein both commonly are contained in a color video signal reproduced off-tape. The technique thus enables precisely treating the high frequency repetitive velocity errors separately, while also treating the random velocity errors, with velocity correction techniques particularly adapted to each type of error. More particularly, the invention provides for separating impact or repetitive velocity errors from random velocity errors by utilizing to advantage the periodic nature of the repetitive velocity errors, that is, the characteristic that the sampled repetitive errors are coherent with vertical sync of the video signal on a frame-by-frame basis. The combined velocity errors in each line of a frame of video are successively averaged together whereby, by their nature, random errors tend to cancel while repetitive errors are enhanced to make them readily available for separation.
To this end, measured velocity errors enter an averaging circuit at horizontal rate as a combined random and repetitive error signal, whereby the circuit maintains an error average for each line of a frame of video. The average is formed by a weighted sum of the error of a given line and the average for the given line over all previous frames. Thus velocity errors for respective lines from previous frames are averaged together. The number of frames in the average is related to a weighting constant K, with a value of K=1/32 being typical, to provide a continuous average over approximately thirty frames. Since repetitive errors such as impact errors, add together, while random errors average to zero over time, the output of the averaging circuit contains only the impact related errors. The repetitive velocity errors then are passed to an improved high order velocity compensating circuit which is optimized to handle such high frequency impact errors. In addition, the repetitive error signal is subtracted from the initial combined random and repetitive error signal to supply the purely random error signal, which then is supplied to a conventional first or second order velocity compensating circuit for correction. The outputs from the repetitive error and the random error compensating circuits then are summed to form a total combined velocity error correction signal. The latter signal is used, in the first type of time base corrector of previous mention, to adjust the clock of an A/D converter in the time base corrector which samples the off-tape video signal, to thereby compensate for offtape phase errors as further described below.
The invention further contemplates supplying curvature-predicting information of the high frequency repetitive velocity errors at not only the color bursts at the beginning and end of a horizontal line, but also at the middle of the line, whereby velocity errors which occur along the middle thereof may be more accurately predicted and thus corrected. To this end, the present technique exploits to advantage the characteristics of impact (and other repetitive) velocity errors, namely, that they not only are vertically synchronous but further that they interlace at frame rate. More particularly, averaged velocity error information from two interlaced fields are combined to generate error samples at the horizontal scan rate at the middle as well as the ends of a scan line which is being corrected. In effect, velocity error samples from one field are used to predict the velocity error of a line in another field. Thus errors which occur rapidly in the middle of a scan line are predicted with an accuracy not previously available, whereby the velocity errors may be corrected with corresponding accuracy.
More particularly, the averaged repetitive velocity errors in each line of a video signal are sampled at horizontal rate. Since the fields of a frame of video are interlaced and since repetitive errors are synchronous with vertical in each field, it follows that the same basic repetitive error profile occurs in both fields of a frame, and samples from a previous field interlace with respect to the samples from a present field, for adjacent lines of video. It follows that the curvature and extent of a repetitive velocity error in a line of the present field now may be more precisely predicted with data taken from the corresponding curvature of the repetitive velocity error in the adjacent line of the previous field. Thus, instead of taking prediction data from only successive color bursts of a horizontally extending series of lines in the same field of a frame, the invention contemplates obtaining the prediction data primarily from the most vertically adjacent bursts of the adjacent line in the previous field of the frame.
To this end, the repetitive velocity errors which are provided by the averaging circuit of previous mention are supplied at horizontal rate to a 1-field delay and to a frequency accumulator circuit. The repetitive velocity error signal is composed of information from a present field, for example, field 2 of a frame, and represents the frequency change that must be added to the present sampling clock oscillator frequency to match it to the off-tape frequency. The frequency accumulator circuit provides at its input a first order error correction signal from two bursts of the present field 2, which error correction signal comprises the present frequency control signal fed to the clock oscillator and which stays constant over each scan line. The delayed signal from the 1-field delay is derived from burst information taken from a previous field, for example, field 1 of the frame. The delayed signal from the 1-field delay is multiplied by a constant in order to convert velocity error in degrees of subcarrier phase to frequency error in Hertz, and the resulting signal then is integrated to generate a horizontal rate ramp with a frequency slope value related to the curvature of the velocity error. The ramp signal is added to the present clock oscillator frequency supplied by the frequency accumulator circuit, to supply the total repetitive frequency control signal. The latter signal then is summed with the frequency control signal generated by the random velocity compensating circuit of previous description to provide a combined frequency control signal whose frequency changes linearly along the line to compensate for corresponding velocity errors in the off-tape signal.
In situations where repetitive velocity errors may be the primary errors of concern, the invention contemplates the separation of the repetitive errors via the frame averaging technique and the subsequent generation of the repetitive velocity error compensating signal via the error interlace technique, to perform repetitive velocity error compensation on the reproduced signal. Conversely, the invention contemplates subtracting the repetitive velocity errors derived via the frame averaging technique to supply the purely random velocity errors, with the subsequent generation of the random velocity error compensating signal to perform random velocity error compensation on the signal. Still further, the variously generated random and/or repetitive velocity error compensating signals may be applied to the A/D converter to correct velocity errors in the off-tape signal prior to the time base corrector, or may be applied to the D/A converter to correct velocity errors on the reference clock side of the time base corrector, depending upon the type of time base corrector in which the invention is being used.
Although the invention is described in arrangements for correcting velocity errors that occur in color television signals reproduced from a magnetic medium, the invention is useful for correcting comparable time base errors in other information signals containing a time base reference signal component that permits the time base of the information signal to be measured periodically.
The invention technique herein is implemented in a digital hardware/software configuration by way of example only, but may be implemented as well in digital hardware only, or in an analog/digital hybrid configuration, as is readily apparent from the description hereinafter.